A cow eats grass and plants so it means that a cow is herbivore...Example -:Cow,Horse etc.Have you notice?That the word Herbivore is similar to grass,shrubs or herbs...Just see the words Herbi and Herbs they are similar to each other...

RHIZOBIUMRhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. ... The bacteria colonize plant cells within root nodules, where they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase and then provide organic nitrogenous compounds such as glutamine or ureides to the plant.

Herbivorous means who eats plants and stems (etc) of plants only. cow eat only plants and stems(etc) of plants right. so cow is a herbivorous animal. then tiger,tiger eats only flesh of other animals they don't eat plants. so they are called carnivorous animals. then crow they eat both flesh and plants. so they are called omnivorous. then dog they also eat both flesh and plants. so they are also a omnivorous animal. so,last and finally there is only one animal that is herbivorous. that animal is cow right. so the answer is cow. thats all.

Cow , as we all know that the cow is a animal while only eats grass and milkmade products and is also a Herbivore.HERBIVORE-the animal which only eats grass and milkmade products and they are also a HERBIVORE.and in the above list tiger,crow and dog are not considered as a Herbivore because the are flesh eater and are also called as CARNIVORE.CARNIVORE-animals which eat other anals and are flesh eater are called CARNIVORE.

Actually there is no need to explain this answer because obviously (cow) is herbivores organism means it eats only grass .plants. leaves. and the remain veg things and others are carnivorous and omnivorous mean they eat meat. flesh or we can say they eat both non-veg or veg. things